This invention pertains to devices for analyzing the frequency spectrum of radar signals. More generally, this invention pertains to devices which use acousto-optic techniques for spectrum analysis.
Acousto-optic devices have been used in the prior art for the interferometric analysis of the frequency spectrum of radar signal. FIG. 1 depicts such a prior art device made of discrete optical components. In the prior-art device, laser 1 generates radiation at optical frequencies, which radiation passes through lenses 2 and 3, which lenses serve to expand and collimate the beam of optical radiation. The collimated beam 4 which emerges from lens 3 then passes through cylindrical lens 5 which serves to focus the beam along a line in the Bragg cells 9 and 12. Beam 4 is split into first and second parts by beam splitter 6. The first part 7 of the beam is reflected by mirror 8 through optical Bragg cell 9. A time-varying reference signal, which typically is a chirped sinusoid (i.e., one that is scanned linearly with time through the frequency range over which the signal is to be analyzed) is applied to Bragg cell 9 so as to create an acoustic signal which propagates through the cell transversely to beam 7. The acoustic wave in cell 9 then interacts with beam 7 and phase modulates the wavefront of beam 7. The second part 10 of the beam produced by beam splitter 6 is reflected by mirror 11 into Bragg cell 12. The time-varying signal to be analyzed is introduced as an acoustic wave in cell 12. The acoustic wave travels transversely to beam 10 and phase modulates the wavefront of beam 10. Parts 7 and 10 of the beam are then recombined by combiner 13.
The phase modulation of parts 7 and 10 of the beam by Bragg cells 9 and 12 produces beams propagating in slightly different directions from that of the unmodulated components. The differences in the directions are dependent upon the frequencies of the modulating signals applied to the Bragg cells. The modulated components of the combined parts of the beam then pass through lens 16 whose position and focal length are selected so as to effect the Fourier transform of the modulated components at the point where the latter impinge on the array of photo detectors 14. Cylindrical lens 15 focuses the transformed components on the array.
The wavefront of part 7 of the beam, after modulation in cell 9, after reflection and combination by combiner 13, and after transformation by lens 16 provides a reference signal at optical frequencies in each of the elements of array 14. This reference signal then interacts or mixes with the component of part 10 of the beam that has passed through cell 12 so as to translate the optical frequencies in part 10 to relatively low frequencies to which each detector can respond. The responses of the individual detectors in the array then represent the frequency components of the signal applied to cell 12.
Although such an interferometric acousto-optic spectrum analyzer has been fabricated and operated using discrete optical components, such a device is a bulky construction, highly sensitive to mechanical vibrations and mechanical distortions, thus limiting the usefulness of the prior art device.